P-Type Semiconductor Zinc Oxide Films Process for Preparation Thereof, and Pulsed Laser Deposition Method Using Transparent Substrates

ABSTRACT

A p-type semiconductor zinc oxide (ZnO) film and a process for preparing the film are disclosed. The film is co-doped with phosphorous (P) and lithium (Li). A pulsed laser deposition scheme is described for use in growing the film. Further described is a process of pulsed laser deposition using transparent substrates which includes a pulsed laser source, a substrate that is transparent at the wavelength of the pulsed laser, and a multi-target system. The optical path of the pulsed laser is arranged in such a way that the pulsed laser is incident from the back of the substrate, passes through the substrate, and then focuses on the target. By translating the substrate towards the target, this geometric arrangement enables deposition of small features utilizing the root of the ablation plume, which can exist in a one-dimensional transition stage along the target surface normal, before the angular width of the plume is broadened by three-dimensional adiabatic expansion. This can provide small deposition feature sizes, which can be similar in size to the laser focal spot, and provides a novel method for direct deposition of patterned materials.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11/405,020 filed Apr. 17, 2006. The above-noted application isincorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

This invention relates to the fabrication of semiconductor ZnO (zincoxide) for application in electric and optoelectric devices.Particularly, this invention provides a simple and reproducible processto dope ZnO to make this material have a p-type conductivity. Thisinvention is also related to the process of pulsed laser deposition oftransparent thin films, particularly to the deposition of large areatransparent thin films and multilayer periodic dielectric structures ontransparent substrates.

2. Description of the Related Art

ZnO is a semiconductor material with a large direct band-gap of 3.37 eVat room temperature. Due to this large band-gap and a large excitonbinding energy (60 meV), ZnO has great potential for use inshort-wavelength optoelectronic devices, such as light-emitting diodes(LEDs), laser diodes (LDs), and ultraviolet light detectors. In the pastyears, this field has been dominated by other materials such as GaN andSiC, which are very expensive to fabricate. In comparison, the cost ofmaking ZnO is very low. For this reason, ZnO has also been consideredfor large scale applications such as solid-state lighting, transparentelectronics, flat-panel displays, and solar cells. However, ZnO isintrinsically n-type, and the unavailability of a reliable process tofabricate robust p-type ZnO is the bottleneck in the commercializationof ZnO-based devices.

Previously, nitrogen (N) doping has been the most widely used method toproduce p-type ZnO. However, use of this method involves a compromisebetween nitrogen solubility and film structural quality. This is becausehigh structural quality requires high growth temperatures, whereas thenitrogen solubility decreases with growth temperature.

WIPO publication WO0022202 provides a Ga and N co-doping approach toachieve p-type conductivity in ZnO with a high N concentration at thesubstitutional sites for oxygen (and therefore a high holeconcentration). However, the results of a few other attempts (K.Nakahara et al., Journal of Crystal Growth, Vol. 237-239, 503, 2002; M.Sumiya et al., Applied Surface Science, Vol. 223, 206, 2004) using thisco-doping method are inconsistent and irreproducible. More recently, aprocess called ‘temperature-modulated growth’ was disclosed in WIPOpublication WO05076341. This method deals with the mutual exclusivitybetween N solubility and film structural quality by periodically fastramping the growth temperature, which, in practice, is a verycomplicated process.

US 2005/0170971 discloses a method of fabricating p-type ZnO film byco-doping N with alkali metal elements. It is believed that theco-doping with the alkali metal atoms leads to the compensation of donordefects in ZnO film and eventually enhances the p-type conductivity.

In the above N doping approaches, either gas sources, such as NO and NO₂(U.S. Pat. No. 6,908,782) or plasma sources that discharge N₂, N₂O, NOor NO₂ gases are employed. However, using nitrogen oxide (NOx) gassesinevitably results in negative environmental impacts. In addition, thereare technical shortcomings in N doping, which are addressed in, e.g., E.C. Lee et al., Phys. Rev. B Vol. 64, 085120, 2001; and H. Matsui, etal., J. Appl. Phys. Vol. 95, 5882, 2004. For example, nitrogen-relateddonor defects can be generated in the doping process due to thecompetition between electron-impact and gas-phase reactions, which oftenoccur within the plasma source and during the growth.

In addition to nitrogen, other group-V elements, such as phosphorus (P)and arsenic (As) have also been used as alternative dopants (K. K. Kim,et al., Appl. Phys. Lett. 83, 63, 2003; Y. R. Ryu, et al., Appl. Phys.Lett. 83, 87, 2003; D. C. Look et al., Appl. Phys. Lett. 85, 5269, 2004;U.S. Pat. No. 6,610,141). However the reported results have not beenwidely confirmed.

The invention uses pulsed laser deposition to grow the ZnO films andfilms of other materials. Pulsed laser deposition (PLD) is a powerfultool for growth of complex compound thin films. In conventionalnanosecond PLD, a beam of pulsed laser light with a typical pulseduration of a few nanoseconds is focused on a solid target. Due to thehigh peak power density of the pulsed laser, the irradiated material isquickly heated to above its melting point, and the evaporated materialsare ejected from the target surface into a vacuum in a form of plasma(also called a plume). For a compound target, the plume contains highlyenergetic and excited ions and neutral radicals of both the cations andthe anions with a stoichiometric ratio similar to that of the target.This provides one of the most unique advantages of PLD over theconventional thin film growth techniques such as chemical vapordeposition (CVD) and molecular beam epitaxy (MBE). The characteristicsof this growth method have been reviewed in several recent journalarticles and are summarized in the book by Chrisey and Hubler. See,e.g., P. R. Willmott and J. R. Huber, Pulsed Laser Vaporization andDeposition, Review of Modern Physics, Vol. 72 (2000), pp 315-327; J.Shen, Zhen Gai, and J. Kirschner, Growth and Magnetism of Metallic ThinFilms and Multilayers by Pulsed Laser Deposition, Surface ScienceReports, Vol. 52 (2004), pp 163-218; and D. B. Chrisey and G. K. Hubler,Pulsed Laser Deposition of Thin Films, John Wiley & Sons, Inc., NewYork, 1994.

With the appearance of commercially available ultra-fast pulsed lasers(with typical pulse durations of a few picoseconds down to tens offemtoseconds), ultra-fast PLD has attracted much attention. First, dueto the extremely short pulse duration and the resultant high peak powerdensity, multiphoton excitation of free carriers becomes significant intransparent materials, and the critical fluence of ablation can bereduced by 1-2 orders of magnitude compared with conventional nanosecondlaser ablation. As a result, the commonly favored ultraviolet wavelengthin nanosecond laser ablation is no longer a requirement in ultra-fastPLD. In fact, focused ultra-fast pulsed infrared lasers have beensuccessfully used to ablate wide band gap materials. Second, when thelaser pulse duration is shorter than the time scale of carrier-phononinteraction (typically a few picoseconds), heat diffusion in the targetis negligible. For this reason, ultra-fast PLD has been considered as agood solution to the problem of droplet generation that has long beenhindering wider application of PLD. Another advantageous result of thelimited heat diffusion is that the removal of target materials isconfined to the area within the laser focal spot. This mechanism hasenabled the precise laser machining with submicron resolution usingultra-fast lasers.

With the advantages of PLD, especially those related to the ultra-fastPLD, in this invention, we also consider the application of PLD in thefield of direct deposition of patterned structures. There have beenseveral types of direct writing techniques involving application ofpulsed lasers. (Here ‘writing’ means either adding materials ontosubstrates, i.e., deposition, or removing materials from substrates,i.e., etching.) For writing patterned materials by means of deposition,laser chemical vapor deposition (LCVD) utilizes laser-enhanceddecomposition of CVD precursors for metals, and can be used fordepositing metal lines and dots. Another technique is the laser-inducedforward transfer (LIFT). See J. Bohandy, B. F. Kim, and F. J. Adrian,Metal Deposition from a Supported Metal Film Using an Excimer Laser,Journal of Applied Physics, Vol. 60 (1986), pp 1538-1539.

In LIFT, a metal thin film is first coated on one side of a transparenttarget substrate. A pulsed laser beam is incident from the other side(i.e., the uncoated side) of the target substrate and is focused on thefront side (i.e., the coated side). The laser ablates the metal film andtransports the metal vapor to the surface of a receiving substrate,which is positioned very close to the target substrate (10 μm or less).Various forms of LIFT have been proposed and are described in severalU.S. patents cited or referenced by this application.

There are a few limitations in the application of the above twotechniques. LCVD involves a complex CVD system and toxic metalorganicgases. In LIFT, the thin metal film limits the amount of material thatcan be deposited. Also, because the metal thin film is supported by atarget substrate, ablation of the target substrate surface that is indirect contact with the film can be involved, which contaminates thedeposited material. Finally, both techniques are suitable only fordeposition of metals.

In order to transfer other types of material, there is known a variationof LIFT, matrix-assisted pulsed laser evaporation (MAPLE) and directwrite, in which the material to be transferred is mixed with a matrixmaterial which is volatile and easy to be ablated and pumped away. SeeP. K. Wu et al., Laser Transfer of Biomaterials: Matrix-Assisted PulsedLaser Evaporation (MAPLE) and MAPLE Direct Write, Journal of AppliedPhysics, Vol. 74 (2003), pp 2546-2557. The mixture is then coated on thesupporting target substrate as in the LIFT method. The MAPLE method issuitable for transferring bio-materials without destroying thebiomolecules. For dielectrics, the deposits often remain in theiroriginal powder form, and adhesion and purity can be problematic due tothe non-epitaxial nature and contamination by the matrix material,respectively.

Other laser-assisted direct depositing techniques include laser ink jetprinting and Micropen© techniques. Both are wet techniques (i.e.,involving liquid binders) and are not suitable for electronic andphotonic applications. Therefore, for direct deposition of patternedhigh purity dielectric materials, in particular, by the way of growth(e.g., epitaxy), a suitable method is still lacking.

SUMMARY OF THE INVENTION

One object of this invention is to provide a simple and reproducibleprocess to fabricate p-type ZnO film with high carrier concentration andhigh conductivity. This method uses two dopant elements to achieve thisobject.

According to theoretical predictions, both group-I and group-V elementsare in principle possible candidates for p-type dopants in ZnO. On theother hand, ZnO is naturally an n-type material, which means that thereis a large number of native donor defects to overcome in order to makeit p-type. In practice, mono-doping ZnO with Li only results insemi-insulating ZnO, which is due to self-compensation of Li-relateddonor defects; and mono-doped ZnO with group-V elements is oftenunstable, mostly due to the low solubility of those elements anddopant-induced donor defects. In this invention, p-type ZnO is producedby doping the material simultaneously with both Li and P. Two possiblereasons for the success of this co-doping method are that Li-related andother natural donor defects are possibly neutralized with co-doping of Pand Li; and second, with Li substitution of Zn, P atoms tend to occupymore oxygen sites, which are desired for the formation of acceptors.Therefore, Li and P benefit each other in playing the role of acceptors.

The invention uses pulsed laser deposition (PLD) to grow the p-type ZnOmaterial. In this method, a pulsed laser beam is focused onto a solidtarget of ZnO mixed with compounds containing both Li and P. Due to thehigh power density of the focused laser pulses, the material on thetarget surface is ablated and a plasma is formed, which in turn isdeposited onto the substrate surface. Both the target and the substrateare installed in a high vacuum chamber with feedthroughs controllingtheir movements.

The most widely used pulsed laser source in PLD is the excimer laser,which has a pulse width of a few nanoseconds (ns) and a wave length inthe UV region. Typical fluence (energy area density) is a few J/cm² fora typical focal spot of about 10 mm². One shortcoming of nanosecondlaser PLD is generation of large droplets with sizes on the order ofmicrons. This has impeded the wide application of nanosecond PLD inindustrial production.

This invention uses a femtosecond or similar ultrashort pulse laser asthe energy source for ablation. Compared with nanosecond laser pulses,femtosecond to picosecond laser pulses have much higher peak power dueto their ultrashort pulse duration, and the ablation mechanism is alsointrinsically different from that of nanosecond laser ablation. Onefundamental difference is that within the femtosecond pulse duration,heat conduction in the target material is negligible, and therefore theablation basically occurs in a non-melting regime. (D. Linde, et al,Applied Surface Science, 109/110, 1, 1997; E. G. Gamaly, et al, AppliedSurface Science, 197-8, 699, 2002; Z. Zhang, et al, Journal of AppliedPhysics, 92-5, 2867, 2002). Consequently in femtosecond PLD, dropletfree growth of thin films can be obtained.

To incorporate different dopant elements into the film, this inventionuses a solid target made of ZnO powders mixed with different impuritycompounds. Using such a mixed solid target largely simplifies the dopingprocess in growth. For example, one can simply choose compounds thatcontain the desired dopant elements. And the dopant concentration can becontrolled by varying the weight percentage of the impurity compound. Inparticular, one of the novel aspects of this invention regarding makingthe target is to mix ZnO powders with lithium phosphate (Li₃PO₄) powderswhich naturally contain both Li and P dopants. Compared with othermethods of doping that use gas sources (for example precursors in CVD)and evaporation sources, this method is easy and cost-effective toperform, and generates negligible negative environmental effect.

Another object of the invention is to provide a method for pulsed laserdeposition of transparent thin films and direct deposition of multilayerperiodic structures on transparent substrates. The setup here includes apulsed laser source, a substrate that is transparent to the wavelengthof the pulsed laser, a continuous wave (CW) infrared laser for heatingthe substrate by irradiation, and a multi-target system. The pulsedlaser is incident on the reverse side of the substrate and directedthrough the substrate and focused onto the target. The ablated materialfrom the target is deposited on the substrate's front face which isfacing the target. The distance between the substrate and the target isvariable by translating the substrate towards or away from the target.When the substrate is away from the target, large area thin films can begrown. When the substrate is very close to the target, due to the smallsubstrate-target distance and the narrow angular distribution of theablation plume at its root, small features can be grown on the substratewith sizes similar to the laser focal spot. By laterally translating thesubstrate, patterned structures (e.g., periodic lines, grids, and dots)can be grown. Multilayer periodic dielectric structures can be grown byalternating the two growth processes at long or short substrate-targetdistances with different target materials, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the pulsed laser deposition systemfor the fabrication of ZnO thin-films in the present invention.

FIG. 2( a) shows X-ray θ-2θ diffraction patterns of a P—Li-co-doped ZnOfilm deposited on a sapphire (0001) substrate.

FIG. 2( b) shows a pole figure measurement of the film.

FIG. 3 is an SEM image of a P—Li-co-doped ZnO film surface.

FIG. 4 shows the transmission spectrum of a P—Li-co-doped ZnO film.

FIG. 5 shows SIMS depth profiles of P, Li, and Al in a P—Li-co-doped ZnOfilm grown on sapphire (0001).

FIG. 6 schematically illustrates the setup for pulsed laser depositionof thin films where the pulsed laser 1 is guided through the back of thesubstrate 3. The substrate 3 can be translated by an x-y-z moving system6. The heating of the substrate is achieved by irradiating the substratewith a CW infrared laser 7.

FIG. 7 depicts the geometry for depositing small size features bypositioning the substrate close to the target such that the depositedfeatures 8 can be of the similar size to the laser focal spot 9. Smallarrows indicate the direction of the plume expansion.

FIG. 8 schematically illustrates the time evolution of the shape of alaser ablation plume, where part (a) indicates the laser focal spot,which also contains the evaporation area; part (b) illustrates theone-dimensional (1D) transition stage (Knudsen layer); and part (c)illustrates the 3D adiabatic expansion stage. Arrows in the figureindicate the directions of plume expansion.

FIG. 9 schematically illustrates a deposition scheme in which thesubstrate is positioned near the target for depositing small sizefeatures and then moved away from the target to deposit intermediatelayers. This process can be repeated to deposit multilayer structures.

Table 1 lists the electrical properties of P—Li-co-doped ZnO filmobtained under different growth and post-growth treatment conditions.

Table 2 compares the electrical properties of an undoped and P-doped ZnOsamples grown under similar conditions as those listed in Table 1.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention first provides an economic and reliable method forfabricating p-type semiconductor ZnO films. The fabrication procedureincludes making of the ablation target, vacuum laser ablation of thetarget and thin film deposition, and post-growth annealing. The setup ofa femtosecond pulsed laser deposition (fs-PLD) system is schematicallyshown in FIG. 1.

The target for laser ablation is made of a mixture of ZnO powders withlithium phosphate (Li₃PO₄) powders of up to 2 wt %. Using lithiumphosphate is an efficient way of simultaneous introduction of the twodopants. The mixed powders are first compressed with a hydrauliccompressor with a pressure of 2-6 ton/cm². The solid target disk is thensintered for 10 hrs at temperatures up to 1000° C. During sintering, thetarget is buried in ZnO powders to avoid decomposition. The target thusmade has a density of 90%. A high density (at least 80%) target ispreferred for reducing particles in the ablation plume, which cause thedeterioration of film quality, i.e. crystallinity and surfacemorphology, especially in the case of fs-PLD. Before introduction intothe growth chamber, the target surface is also polished. The targetmanipulator provides lateral and rotational movements in the targetsurface plane. The target holder carousel has four stations to holddifferent targets so that multilayer films can be grown with differentchemical compositions.

A femtosecond pulsed laser was used for the PLD process to fabricate theZnO film. The laser beam has a pulse width in the range of 10 fs to 1 psand a pulse energy in the range of 2 μJ to 100 mJ. The beam is firstexpanded by 10 times by a telescope and then focused onto the targetsurface by a focusing lens. Under tight focus, the fluence (energydensity) at the focal spot can be varied up to 250 J/cm² with a spotsize of 400 μm². Due to the extremely high peak power (>5×10⁶ W) of theultra-short pulse, the threshold of ablating ZnO using a femtosecondlaser is relatively lower compared with that of nanosecond pulsed laser.A fluence above 1 J/cm² is sufficient to ablate the ZnO target andgenerate an ablation plasma. However, a high fluence up to 5 J/cm² ispreferred to reduce the number of particles in the plasma plume.

Note that other pulsed lasers such as nanosecond or picosecond laserscan also be used in the PLD (pulsed laser deposition) process, and otherhigh energy sources such as electron beam or ion/plasma can also be usedfor the purpose of evaporating or sputtering the solid targets.Therefore the preferred high energy source is a femtosecond to lowpicosecond pulsed laser for the reasons elaborated above, although theinvention is not so limited.

The substrates are mounted on a substrate heater which can heat thesubstrates up to 900° C. The substrate manipulator provides for lateraland rotational movement to the substrate in its surface plane. Thedistance between the substrate and the target is also adjustable usingthe substrate manipulator.

The vacuum system is pumped by a turbo-molecular pump and operates at abase pressure of 1.5×10⁻⁸ torr. The chamber can also be back-filled withdifferent gases via gas inlets and outlets (not shown in FIG. 1) duringgrowth. During the growth experiments of this invention, the chamber isfilled with 0.1-20 millitorr oxygen.

Laser ablation occurs when the laser beam is focused on the targetsurface. During PLD growth, the laser focal spot is fixed while thedisk-shaped target is rotated around its surface normal axis andlaterally translated back and forth along its surface. This isequivalent to scanning the laser beam across the target surface. Theangular velocity of the rotation is 1 rev/sec. The lateral translationalmovement speed is 0.3 mm/s. The fluence is 20 J/cm⁻². The pulserepetition rate is kept at 1 kHz. These parameters are all variable andthe invention is not restricted to these values, although the givenvalues have been found to be nominal for the growth described herein.

Before film deposition, the substrate is first outgassed by heating attemperatures up to 600° C. The substrate is then treated with an oxygenplasma for about 5 min to remove hydrocarbon contaminants. The targetsurface is usually pre-ablated for at least 20 min before filmdeposition. The purpose of pre-ablation is to clean the target surface,which is usually contaminated during fabrication. During thepre-ablation, a shutter is inserted between the target and the substrateto protect the substrate surface.

The growth is started by first depositing undoped ZnO for about 20 minat a low temperature of 300-400° C. The thickness of this lowtemperature layer is about 30 nm. For heteroepitaxy on sapphire, thisearly growth stage provides a buffer layer to absorb the strain due tothe large lattice mismatch between ZnO and sapphire. After the bufferlayer growth, the undoped ZnO target is moved away and the Li—P-co-dopedZnO target is moved into the ablation position and growth of the dopedlayer is started. The substrate temperature is also elevated to a higherrange between 450-700° C. The higher growth temperature enhances atomsurface mobility, which gives a better film crystallinity. The totalthickness of the film is greater than 300 nm (up to 1 micron). Aftergrowth, the sample slowly cools down in an oxygen background of a fewtorr.

Post-growth treatment primarily includes annealing in a tube furnaceunder ambient conditions. Annealing temperature ranges between 500-1000°C., preferably between 600-800° C. Total time of annealing is typicallyin the range of 2-60 minutes. The annealing can also be performed insitu in the growth chamber after growth, provided that the growthchamber is backfilled with oxygen. However, ex situ annealing in a tubefurnace is preferred because of the available high annealing temperatureand fast heating and cooling rates.

The crystalline structures and electrical and optical properties of thefilms are examined by X-ray diffraction (XRD), Hall, and transmissionmeasurements. The dopant depth profile is examined by secondary ion massspectroscopy (SIMS).

FIG. 2 (a) shows the typical θ-2θ XRD pattern of a Li—P-co-doped ZnOfilm deposited at 400° C. for the buffer layer and 450° C. for the dopedlayer on a sapphire (0001) substrate. The two peaks correspond to thebasal planes of wurtzite structure ZnO and of the sapphire substrate. Nodiffractions from the other planes of ZnO or from impurity phasesrelated to phosphorus and lithium are detected within the detection andresolution limit of the instrument. The full width at half maximum(FWHM) of the (0002) ZnO rocking curve is 0.8°. The epitaxialrelationship between ZnO and the substrate is reviewed to be[2-1-10]_(ZnO)//[10-10]_(sapphire) by pole figure measurements, as shownin FIG. 2( b). This is also the less-strained epitaxial relationshipcompared to the one without the 30° rotation.

The surface morphology of the above sample is shown in FIG. 3. Thesurface morphology suggests that the film is composed of small grains.The average grain size is measured to about 70 nm. FIG. 4 is an opticaltransmission spectrum of the sample. A clear cut-off edge at 370 nmindicates a band gap near 3.35 eV, and the high transmission in thevisible to near infrared region indicates good optical properties.

FIG. 5 shows the SIMS (Secondary Ion Mass Spectroscopy) depth profile ofLi, P and Al impurity in a ZnO film grown on sapphire. The target usedfor this sample has a nominal Li₃PO₄ concentration of 1%. It is observedthat significant amounts of Li (10²⁰ cm⁻³) and P (10²¹ cm⁻³) are presentin the film, which are much higher than the typical dopant concentrationin conventional semiconductors such as Si and GaAs. One reason is thehigh acceptor activation energy in ZnO, which gives a very lowactivation efficiency of less than 0.1%. Another reason is the high Alinter-diffusion from the substrate into the film, which is evident inthe SIMS profile in FIG. 5. Al is an efficient donor for ZnO. Therefore,a large number of acceptors is needed to compensate the Al donor.Converting the Al signal to concentration, it was found the equivalentAl concentration averages 10¹⁸ cm⁻³ in the film. Similarly, GaNsubstrates can have a similar interdiffusion problem, as Ga is also anefficient donor for ZnO. Therefore, by using other substrates such asMgO, SiC, Si, and insulating ZnO, the high concentration of Li and P isnot required, and the weight percentage of lithium phosphate in thetarget can be lowered correspondingly down to 0.01%.

Table 1 lists the results of Hall measurements on a few Li—P-co-dopedsamples grown under different conditions, where the post-growthannealing treatments are all performed in a tube furnace in ambientconditions. The target is a mixture of ZnO with 1 wt % Li₃PO₄. TheL-P-co-doped sample annealed at 600° C. and the as-grown sample (notshown in the table) are both weak n-type materials. After annealing at700° C., the sample converts to p-type. Higher annealing temperatures(>900° C.) yield more resistive but still p-type materials.

Table 2 compares the results of Hall measurements on an un-doped ZnO anda P-doped ZnO grown under otherwise similar conditions. The target forthe P-doped version is made of ZnO mixed with 1 wt % P₂O₅. It can beseen that P—ZnO remains a strong n-type material after differentpost-growth treatments. (As-grown P—ZnO is also of strong n-type.)

Thus it is seen that the co-doping technique of the invention results instable p-type ZnO which cannot be otherwise obtained.

This invention also provides a method especially adapted for growingtransparent thin films on transparent substrates. The setup isillustrated in FIG. 6. The substrate is positioned at a distance fromthe target. The pulsed laser is guided in such a way that it is incidentfrom the back of the substrate, which is transparent, and then focuseson the target. Heating of the substrate, if necessary, can be providedby, for example, a CW infrared laser, e.g., a CO₂ laser (wavelength of10.6 μm), which can be strongly absorbed by most dielectrics. Thesubstrate can be laterally and vertically translated by an x-y-ztranslation system.

Because in this geometry, the substrate and the deposited material needto be transparent to the pulsed laser wavelength, this setup excludesopaque substrates and deposited materials. For the commonly used nearinfrared pulsed laser wavelengths e.g., 800 nm of Ti-Sapphire and 1 μmof Nd:YAG, most dielectrics are transparent and therefore can be used asthe target material. These include, but are not limited to alumina(Al₂O₃), silica (SiO₂), metal oxides (MgO, ZnO, TiO₂, ZrO₂, Nb₂O₅,)including transparent conductive oxides (TCOs), such as In—Sn—O, F—Sn—O,Nb—Ti—O, Ga—Zn—O, Al—Zn—O, and p-type delafossite oxides CuM(III)O₂,(M(III)=Al, Ga, In), and wide gap III-V and II-VI semiconductors andtheir alloys (GaN, AlN, ZnS, ZnSe, ZnTe). The substrates can betransparent insulators such as, but not limited to, sapphire, quartz,glass, and transparent polymer substrates.

Because the substrate can be laterally translated in the x-y plane, thissetup also provides a solution to scale up PLD for large area thin filmdeposition. By simply alternating targets of different materials,multilayers of thin films can also be grown.

FIG. 7 illustrates the situation when the substrate is positioned veryclose to the target. The purpose of bringing the substrate close to thetarget is to obtain small deposition features. A few issues need to beconsidered for this purpose.

The first important aspect is the shape of the ablation plume. Ingeneral, the angular distribution of the plume is described asf(θ)=cos^(n)(θ), where θ is the angle formed with the target surfacenormal, and n is a number depending on the ablation parameters. Theangle at which the plume intensity is half of its maximum value istherefore expressed as θ_(1/2)=cos⁻¹(2^(−n)). For nanosecond ablation, awide range of the value of n has been reported varying from about 3 toabove 20, depending on the pulse duration, pulse energy, wavelength, andthe type of target material (i.e., transparent or opaque). Recently, ithas been found that for the same target material and similar fluences,the plume of ultra-fast laser ablation is much narrower than that ofnanosecond laser ablation. For example, for metal targets, a typicalnanosecond laser ablation plume has a θ_(1/2) of about 33°(corresponding to n ˜4), while for picosecond and sub-picosecond laserablation, the θ_(1/2) value is only 20° (corresponding to n˜10). See R.Teghil et al., Picosecond and Femtosecond Pulsed Laser Ablation andDeposition of Quasiparticles, Applied Surface Science, Vol. 210 (2003),pp 307-317.

For oxides (e.g., ZnO), a comparison of the femtosecond and nanosecondlaser ablation plume shapes also shows the very narrow angulardistribution (θ_(1/2) less than 20°, judging from the CCD image), i.e.,strong forward peaking of the femtosecond laser ablation plume. See J.Perriere et al., Comparison Between ZnO Films Grown by Femtosecond andNanosecond Laser Ablation, Journal of Applied Physics, Vol. 91 (2002),pp 690-696.

In addition to the above considerations, a careful examination of theearly stages of the evolution of the laser ablation plume suggestsfurther possibilities of reducing the deposition feature size at smallsubstrate-target distances. In laser ablation, due to the boundaryconditions imposed by the target surface, the evaporated gas experiencesa pressure normal to the target surface, which results in a layer of gas(so-called Knudsen layer) expanding one-dimensionally along the surfacenormal. The adiabatic 3D expansion occurs after this layer of gasreaches thermal equilibrium through collision. These stages areschematically illustrated in FIG. 8 (adopted from J. C. S. Kools, E. vande Riet, and J. Dieleman, A Simple Formalism for the Prediction ofAngular Distributions in Laser Ablation Deposition, Applied SurfaceScience, Vol. 69 (1993), pp 133-139). According to the Monte Carlosimulation reported in D. Sibold and H. M. Urbassek, Kinetic Study ofPulsed Desorption Flows into Vacuum, Physical Review A, Vol. 43 (1991),pp 6722-6734, at high ablation rates (>1 monolayer per pulse), thethickness of the Knudsen layer is about 20λ, where λ is the mean freepath. Under typical nanosecond laser ablation conditions, this thicknessis on the order of microns. By lowering the fluence and thereforelowering the ablation rates below 0.1 monolayer per pulse, the Knudsenlayer can be thicker (due to reduced collisions), i.e., up to 100 μm. Inaddition, because ultra-fast laser ablation can generate faster atomsand ions than nanosecond laser ablation at similar fluences, thisthickness can be larger in ultra-fast laser ablation due to theincreased velocity of ablated atoms and ions. On the other hand, it isobserved in experiments that under typical nanosecond laser ablationconditions, 3D expansion occurs after a relatively long time scale of300 ns. See: Z. Zhang, P. A. VanRompay, J. A. Nees, and P. P. Pronko,Multi-diagnostic Comparison of Femtosecond and Nanosecond Pulsed LaserPlasmas, Journal of Applied Physics, Vol. 92 (2002), pp 2867-2874; andR. Gilgenbach and P. L. G. Ventzek, Dynamics of Excimer Laser-AblatedAluminum Neutral Atom Plume Measured by Dye Laser Resonance AbsorptionPhotography, Applied Physics Letters, Vol. 58 (1991), pp 1597-1599.Assuming a typical drift velocity of 500 m/s for the Knudsen layer, thistime scale suggests a thickness of 150 μm.

Because the above-estimated thickness of the 1D expansion layer, on theorder of 100 μm (preferably under low fluences) is a practicallyachievable substrate-target distance, we can utilize the narrow angulardistribution of the ultra-fast laser ablation plume to achievedeposition of small patterns with sizes close to the laser focal spot bypositioning the substrate very close to the target, as illustrated inFIG. 7. Note that the typical size of laser focal spot can be made verysmall (from a few microns to submicron diameter) by using beam expansionand a focusing lens of a large numerical aperture. Also note that theevaporation zone is actually contained within a smaller diameter thanthe laser focal spot in ultra-fast laser ablation (due to the limitedheat diffusion). This deposition process can serve for direct writing ofmaterials onto a substrate. In addition, by using high repetition rate(typically MHz) ultrafast lasers, a high deposition rate can also beachieved.

With the above-introduced capability of precise deposition ofmicron-scale features, two-dimensionally patterned structures such asarrays of dots and lines can be obtained simply by positioning thesubstrate close to the target and translating the substrate laterally.This process can thus serve as a means of direct writing of materials.

A combination of the two growth processes at long and shorttarget-substrate distances can provide a variety of designed growthpatterns. For example, by alternating the two growth processes at longand short substrate-target distances with different target materials,respectively, lateral (in-plane) periodic structures can be obtained andthen covered with intermediate layers of different materials, asillustrated in FIG. 9. This method would be suitable for fabrication of,for example, photonic devices such as dielectric mirrors, e.g., Braggreflection mirrors and 2D and 3D PBG materials in the infrared andmicrowave regime.

Although several examples or embodiments have been described, theinvention is not so limited, and is embodied in each novel feature andeach combination of features, which particularly includes allcombinations of the claimed features, even if this feature or thiscombination of features is not explicitly set forth in the claims or inthe specification. In addition, this specification incorporates byreference all combinations set forth in the following claims.

TABLE 1 Electrical properties of Li—P-co-doped ZnO films made with atarget of ZnO:Li₃PO₄ (1 wt %) Growth Anneal Hall Carrier TemperatureTemperature Anneal Resistivity Mobility Concentration Carrier Sample (°C.) (° C.) Time (min) (ohm cm) (cm²/V) (cm⁻³) Type #5720 450 600 5 72 2  4 × 10¹⁶ n #5720 450 700 3 40 1 1.5 × 10¹⁷ p #5715 500 800 3 410 101.5 × 10¹⁵ p #5711 500 800 11 1240 11   4 × 10¹⁴ p

TABLE 2 Results of Hall measurements for an un-doped and a P-doped ZnOfilms Growth Anneal Hall Carrier Temperature Temperature AnnealResistivity Mobility Concentration Carrier Sample (° C.) (° C.) Time(min) (ohm cm) (cm²/V) (cm⁻³) Type un-doped 500 N/A N/A 0.4 22 8 × 10¹⁷n (#50623) P-doped 450 700 3 0.5 2 6 × 10¹⁸ n (#50726) P-doped 450 800 30.2 6 5 × 10¹⁸ n (#50726) P-doped 450 900 3 0.09 18 4 × 10¹⁸ n (#50726)

1. An apparatus for depositing transparent thin films and directdeposition of patterned structures on a substrate, comprising; a pulselaser source, a beam delivery system for guiding said laser through thesubstrate and focusing it on a target, a transparent substrate ontowhich material ablated or evaporated from said target is deposited, anda translation system for moving said substrate relative to said target.2. An apparatus as claimed in claim 1, further comprising a substrateheating system, comprising a CW infrared heating laser.
 3. An apparatusas claimed in claim 1, wherein said target is mounted to a targetholder, and said target holder is a multi-target apparatus capable ofmoving multiple targets successively to a target location.
 4. Anapparatus as claimed in claim 1, wherein said translation systempositions said substrate at a variable distance from said target, saidvariable distance ranging from 10 μm to 30 cm.